CBOT's board has thrown its support behind the combination with CME, the largest U.S. futures mart.

The proposal for the two giant Chicago marts to create the world's largest derivatives exchange was given the green light by the U.S. Justice Department on Monday. Shareholder votes at both companies are scheduled for July 9.

ICE said it intends to solicit votes against the proposed CBOT-CME combination at the CBOT stockholder meeting.

Under Tuesday's revised terms CBOT shareholders would be able to elect up to $2.5 billion in cash in lieu of ICE shares.

ICE reconfirmed the ratio of 1.42 ICE shares for each CBOT share originally announced in mid-March.

ICE also said that it would pay for the entire $294 million break-up fee and expenses that would be payable to CME if CBOT agreed to partner with ICE instead.

Based on Tuesday's closing prices the ICE offer is valued at $211.55 per CBOT share, or about $1 billion in additional value above the CME's offer, which itself was sweetened in May.

CBOT's board has said that a combination with CME represents substantially less "execution risk" and that the risks to its futures trading franchise from a partnership with ICE could be "catastrophic."